In general, input and output devices such as scanners, printers and display monitors respectively have a different range of color capabilities. A particular color is usually defined in a three dimensional color space which includes the tristimulus RGB coordinates and the Lab coordinates. The definitions of these coordinates in pages 585-587 of Computer Graphics by Foley et al. (1995) pages 63-66 of Principles of Digital Image Synthesis by Glassner (1995) are incorporated herein by external reference and are not reiterated here. The LCH coordinates are based upon Luminance (L), chroma (C) and hue (H). A range of colors that a device is capable of processing in a certain color space is known as a gamut. As shown in the Lab color space in FIG. 1, a gamut of a printer is generally smaller than that of a color display monitor or a cathode ray tube (CRT). Because of the difference in the gamut or color space between the devices, when an image defined for one device such as a display monitor is outputted into another device such as a printer, the colors of the printed image fail to exactly match those of the displayed image.
To further illustrate a gamut difference, FIG. 2 shows a cross section of a three dimensionally represented gamut near "red" for a color CRT and an ink jet printer. The inner dotted lines represent a boundary of the color range for the inkjet printer while the outer solid lines represent a boundary for the color CRT and include the inkjet printer gamut. In the Y axis, luminance L is plotted between a point representing white and another point representing black. In the X axis, purity or croma C is plotted for these two devices. Each line indicates an outside boundary of a gamut. Since the gamut of the inkjet is smaller than that of the color CRT, certain CRT colors shown in the area between the two gamut boundaries are not printed by the inkjet printer. In order to accommodate the different gamut sizes, gamut mapping has been considered.
Gamut mapping or color space correction is a technique to fit one gamut of a first size to another gamut of a second size. If a large gamut is mapped into a small gamut size, the mapping process is known as a gamut compression. On the other hand, a small gamut is mapped into a large gamut, the mapping process in known as a gamut expansion. In general, the gamut mapping techniques are classified as either local and global. In a local mapping technique, only colors outside of the smaller gamut is adjusted. In contrast, in a global mapping technique, all colors in the gamut are adjusted. Although the global mapping technique generally shifts all of the colors in the gamut at an equal rate, a hybrid approach between the global and local mapping techniques adjusts certain selected portions of the color space at an equal rate.
In addition to the above described general categories of the mapping techniques, the color correction process involves the independent adjustments of dominant wavelength, luminance and purity. During the mapping process, one or more of the above characteristics is controlled at a constant level so that the mapped colors are finely adjusted.
Furthermore, Japanese Laid Patent Publication 4-119765 discloses a color digital copier system for adjusting a gamut compression technique based upon an input pattern. To adjust the compression, an input image is scanned to generate a color distribution pattern, and the pattern is stored. During the pixel-by-pixel gamut compression of the same image, the compression ratio for a particular pixel color value is adjusted according to the previously stored color distribution pattern. In addition to the adjustment of the mapping ratio, the system also selects a mapping technique based upon the input pattern. Similarly, U.S. Pat. No. 3,133,510 issued to Farber also discloses independent adjustments of purity and luminance depending upon an input image. The above described prior attempts focus upon the input image characteristics to adjust a gamut mapping technique.
Another prior approach in color adjustment is disclosed in U.S. Pat. No. 4,500,919 issued to Schreiber. The Schreiber patent is directed to computer assisted color matching for printing a color image to match original colors. An operator of the system is allowed to modify the colors on a display before actually printing on an image-carrying medium such as paper. After the operator finishes the matching process, the system determines an appropriate amount of toner or ink for printing based upon the previously stored printing ink data. In this system, the color editing is based upon the operator's perception rather than the above described automatic gamut compression technique using the input image characteristic data. Although the Schreiber patent discloses the color adjustments based upon the operator's perception of the output colors, the adjustments are manual and not automatic.
The problems of the above described prior attempts include that the output color image is not used in selecting he most appropriate mapping technique. In Japanese Laid Patent Publication 4-119765 and U.S. Pat. No. 3,133,510, the mapping is adjusted based upon the input color distribution, and no evaluation is made on the outputted colors. Although the Schreiber patent focuses upon the output colors, the adjustment is made manually by an operator. In order to achieve consistently accurate results in gamut mapping, output colors should be mapped in a systematic fashion and the generated output colors should be evaluated in an objective fashion with respect to original input colors.